i dont know about the foreign laps but the early laps were plated on to copper or copper plated disc’s. for these, there not worn out till you see the copper. plated direct on steel probabley not till you see the shiny spot of steel showing through. till then always usable. there are 2 things that can dull down your lap, embedded clogging of material and chattered or rounded diamond. you can usually improve performance by dressing them. for very coarse up to 400 grit use an old 220 grit silicone carbide grinding wheel, use high speed,LOTS of water and presser and this will clean out cuttings and remove a little nickel exposing fresh higher diamond points. fir finer wheels i use a 600 grit silicone carbide stick the same way, also sometimes man made sapphire or glass including obsidian will help improve fine wheel/disc performance. i have some china made laps that both students and i have had stones pop off and dig the lap, do a bit of sapphire on that spot and we’re still using them jut fine.